Hello Jeff, Never did the tests personally but I know from others work where I used to work that putting 2 RF signals through an isolator of significant power will produce measurable intermods and with the powers of a typical transmitter, definitely a problem.
There were problems with just the connectors in a duplex system when the RX and TX paths shared a junction that wasn't perfectly linear (assuming proper duplex filtering as well). The Mill Spec change in the 90s caused grief when they spec'd nickel plating under the gold on the pins! IM levels from that PIM source were -80dBm levels! That is why one must never use Mil Spec connectors if you have IM concerns (unless something has changed recently - out of the loop) In the isolators/circulators, there are components that are magnetic such as the variable capacitors and the metal housings that will never get the device IM free even if the ferro material was perfect. ALSO, for the original poster, there is an IM combination of the 145.25 and 145.05 transmitter frequencies that will produce an IM product on 144.65. Using 2 antennas separated by 50 dB and using low IM components everywhere from the antennas, supporting structure, transmission lines, connectors and filter systems, may prevent the problem. Harold --- In Repeater-Builder@yahoogroups.com, "Jeff DePolo" <[EMAIL PROTECTED]> wrote: > > > Again, the circulator will produce intermod when external strong > > signals enter the antenna port when used as a switch for the duplexer > > and there is transmitter power also going through it. 50 Watts or > > 47dBm to -116dBm receiver sensitivity is 163dBm dynamic range. The > > device is not linear enough to prevent detectable IM when strong > > signals are received that are of the correct frequencies to cause IM > > to a receiver. > > Which was why I was reluctant to go along with the idea of feeding a > receiver off the third port, or even worse, passing multiple high-level > signals through a common circulator. > > Have you ever quantified IMD in a run-of-the-mill VHF/UHF ferrite circulator > before Harold? When I get some time, I plan on trying to do that. I figure > I'll use a high-quality pass/reject duplexer with > 100 dB of isolation as a > two-transmitter combiner, and feed the output to a circulator with the > remaining two ports terminated properly, and measure the third-order > products at both terminated ports. I'll also be curious to see at what rate > the IM products increase as you start to approach (or even exceed) the > isolator's power-handling rating. > > --- Jeff >
